Devices that separate or concentrate particles from an airflow and direct them to a collection medium are known. Examples of such devices are provided in U.S. Pat. Nos. 3,731,464 (Brumbaugh et al.); 4,767,524 (Yeh et al.); and 4,972,957 (Liu et al.).
Such conventional aerosol selection devices separate the particle laden air into two airflows by inertial separation. The larger sized particles are then conducted to a collector that is located externally to the selection device. None of the prior art devices enable the selection and collection to be accomplished in a single simple assembly.
U.S. Pat. No. 4,972,957 attempts to provide such an assembly by including impactor plates within the selector that collect the separated particles. It is still necessary to partially disassemble the apparatus however, in order to remove the impactor plates for their subsequent analysis. It is preferable that the separation, collection and analysis be accomplished in an efficient manner so that subsequent air flows may be analyzed in quick succession.
While it has always been desirable to provide a simplified separation and collection assembly, the need for such an assembly has been highlighted by recent developments in the area of concealed substance detection.
Conventional detection techniques involve the collection of vapors that are emitted from concealed substances and the subsequent identification of such vapors using mass spectrometric, chromatographic, or chemiluminescent techniques. The lower vapor pressures of modern plastic explosives and narcotics such as cocaine or heroin, make their detection by vapor collection very difficult, if not impossible.
Recent developments in the field of contraband detection have been directed toward the analysis of particles that attach themselves to articles such as baggage. Particles of explosives and narcotics are transferred to the surface of these articles by persons who have been handling said explosives and narcotics. It is also recognized that substances having lower vapor pressures nevertheless emit trace amounts of vapor that will agglomerate with dust or other particles that settle upon articles or are transferred to such articles by persons handling the substances. Such particles may be vacuumed from the article and the particles may be collected upon a collection medium. The collection medium may then be heated to release the vapors agglomerated upon the particles collected. The vapors may then be analyzed by mass spectrometry or other techniques to determine their chemical makeup.
One problem faced with such detection techniques is the occurrence of false alarms caused by an abundance of small particles introduced to the collection medium. These may be caused by a general level of contamination by a particular substance, ie. a background effect, rather than being indicative of the presence of a significant quantity of the substance. It has now been discovered that any background effect or incidental contamination will be caused by relatively small particles. If there is any significant quantity of a substance of interest, then it should give off larger particles. Thus, it has now been discovered that by preferentially collecting only larger particles, erroneous readings due to background effects are substantially eliminated. It is therefore desirable that a concealed substance detection apparatus include a particle selector for selecting particles of a desired size.
As previously discussed, conventional particle selectors are not adaptable to large scale detection applications because of their failure to provide a single simple assembly for the selection and collection of particles. Such an assembly is desired in order that articles may be examined in relatively quick succession. To accomplish this, it is necessary that the collection medium be quickly accessed or moved from the collection location to an analysis location.
The present invention therefore provides an assembly for selecting and collecting particles, as well as an apparatus incorporating such an assembly for enabling the analysis of particles in an efficient manner.